Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction

The use of metal-free carbon nitride and light to drive catalytic transformations constitutes a sustainable strategy for organic synthesis. At the moment, enhancing the intrinsic activity of CN catalysts by tuning the interfacial coupling between catalyst and substrate remains challenging. Herein, we demonstrate that urea-derived carbon nitride catalysts with the abundant −NH2 groups and the relative positive charged surface could effectively complex with the deprotonated anionic intermediate to improve the adsorption of organic reactants on the catalyst surface. The decreased oxidation potential and upshift in its highest occupied molecular orbital position make the electron abstraction kinetics by the catalyst more energetically favorable. The prepared catalyst is thus utilized for the photocatalytic cyclization of nitrogen-centered radicals for the synthesis of diverse pharmaceutical-related compounds (33 examples) with high activity and reusability, which shows competent performance to the homogeneous catalysts.

were acquired on JEOL RESONANCE 600M spectrometer. The Fourier transform infrared (FTIR) analyses were recorded on a BioRad FTS 6000 spectrometer.
Fluorescence (PL) spectra and time-resolved PL was measured in Edinburgh FL-FS 920 TCSPC. The electron paramagnetic resonance (EPR) measurements were carried out on a Bruker Model A300 spectrometer. Thermogravimetric analysis (TG) was performed on the Netzsch STA 449F3 simultaneous thermal analyzer. Temperature programmed desorption (TPD) measurements were carried out on a Micromeritics AutoChem Ⅱ 2920 Chemisorption Analyzer.The Brunauer-Emmett-Teller (BET), and surface area (SBET) of the samples were determined by N2 adsorption using a Micromeritics ASAP 2046 N2 adsorption apparatus (USA). All reactions were carried out with dry solvents under an atmosphere of argon using standard Schlenk techniques unless otherwise stated. Column chromatographic purification of products was accomplished using 200-300 mesh silica gel. 1 H and 13 C NMR spectra were recorded on a JEOL RESONANCE 600M spectrometer in the solvents indicated (note: CDCl3 referenced at 7.26 and 77.00 ppm respectively); Coupling constants are reported in Hz with multiplicities denoted as s (singlet), d (doublet), t (triplet), q (quartet) and m (multiplet). High-resolution mass spectra were recorded on Exactive Plus LC-MS (ESI) mass spectrometers.

Electrochemical Measurements.
Electrochemical measurements were conducted with a BAS Epsilon Electrochemical System in a conventional three-electrode cell, using a Pt plate as the counter electrode and an Ag/AgCl electrode (3 M KCl) as the reference electrode. The working electrode was prepared by dip-coating sample slurry on ITO glass and then dried at room temperature.

AQE measurement.
To determine the apparent quantum efficiency (AQE) of g-CN-U, the cycloaddition photoreaction was conducted on the same device at room temperature, using a 420 LED lamp as the light source. The light intensity was measured at 31.7 mW cm -2 and the irradiated area was controlled at 1 cm 2 . After reaction for 1h, the product 1a was 31% yield determined by crude NMR (benzyl oxide as the internal standard). The calculation processes of AQE are shown below: K-PHI-b: 100 mg K-PHI was dispersed equably in 100 mL potassium hydroxide solution (1 M). After stirring for 1 h, the carbon nitride sample was filtrated, rinsed with deionized water, followed by drying at 70 °C under vacuum. This sample is denoted as stirring for 0.5 h, the carbon nitride sample was filtrated, rinsed with deionized water, followed by drying at 70 °C under vacuum. This sample is denoted as H-PHI.

Details of theoretical calculations
All calculations were carried out using density functional theory (DFT) by employing the Vienna Ab-initio Simulation Package (VASP) code. 1 Nuclei and core electrons were described by the projector augmented wave (PAW) potentials. 2 The generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was employed combined with the Grimme's DFT-D3 correction. [3][4][5] A Monkhorst-Pack mesh of 3×3×3 k-points was used in the Brillouin zone for geometry optimizations and electronic structure calculations. The cut-off energy of the plane-wave expansion was set to 550 eV, and the convergence thresholds of the energy change and the maximum force were set to 10 -5 eV and 0.01 eV/Å, respectively. A vacuum space was set to 20 Å to avoid the interactions between neighboring molecules. The Bader Charge Analysis code was used for the population analysis. [6][7][8][9] Supplementary Figure 1. Catalysts NMR characterization. The solid-state 13 C NMR spectra of g-CN-U, g-CN-DCDA and K-PHI. tetrafluoroborate, substrate 1z (2 mM) in CH3CN using a glassy carbon as working electrode, a platinum wire as a counter electrode, a silver wire as pseudo reference, ferrocene as internal standard and reference electrode at 20 mV/s scan rate.
Supplementary Figure 11. Proposed reaction mechanism. Photocatalytic cyclization of nitrogen-centered radicals for dihydropyrazole synthesis over carbon nitride.  The resulting mixture was then degassed via the 'freeze-pump-thaw' procedure (3 times) under argon atmosphere. After that, the solution was stirred at 3 W blue LEDs (420 nm) at room temperature until the reaction was completed (monitored by TLC analysis).

Characterization of Products
17.5 mg in 55% isolated yield,% as a colorless oil. 1